In recent years, devices and applications involving LEDs (i.e., light emitting diodes) are gaining popularity. Such devices and applications range from light sources for general illumination, signs and signals, to display panels, televisions, etc. Regardless of the applications, LED driver circuits are used in supplying power to the LEDs.
An LED panel generally refers to a device that comprises an array of LEDs that are connected together or a plurality of sub-modules, each sub-module having one such LED array. LED panels usually employ arrays of LEDs of a single color or different colors. When individual LEDs are used in certain display applications, each LED usually corresponds to a display pixel. An RGB LED unit refers to a cluster of three LEDs, namely, a red LED, a green LED, and a blue LED. When RGB LED units are used in certain display applications, each RGB LED unit corresponds to a display pixel. Surface mounted RGB LED units usually have four pins, one pin for each of the red, green, and blue LEDs and another pin for either a common anode or a common cathode.
Traditionally, LED arrays are often arranged in a common anode scan configuration, in which the anode of the LEDs are operatively connected to a power source via a switch element while the cathode of the LEDs are tied to the output of current drivers. In such a configuration, an NMOS driver is often used as the current sink. An NMOS is preferable over a PMOS because NMOS has a larger current capacity and a lower Rds(on) for a given design geometry.
In a common anode configuration, all RGB LEDs are connected to the same power supply and have a same supply voltage. As is well known in the art, the red LED forward voltage is significantly lower than that of green and blue LEDs. Using a same supply voltage for the red, green, and blue LEDs requires adjusting the supply voltage to match the forward voltage drop of individual LEDs, for example, by installing a bias resistor between the power supply and the LED. Consequently, a significant amount of energy is released as heat on the bias resistor. For example, if the supply voltage is 5 volts, since the forward voltage drop of a red LED is about 2.0 volts, approximately 60% of the energy is lost as heat on the bias resistor. Such heat generation not only wastes energy, but also complicates the design of driver circuitry, e.g., by increasing the demand for heat removal.
It is often desirable to have a display of high resolution. The smaller the size of a pixel pitch is, the higher the resolution of the display may have. An LED display system has many components, e.g., a constant current driver, a decoder, power MOSFETs to control scan line switching, and biased resistors for some LEDs (such as red LEDs) to reduce LED driver operating voltage. These components are often mounted on a PCB (printed circuit board) as discrete parts, not only increasing the manufacturing cost by increasing the number of layers in PCB, but also making it difficult to curb noise on the PCB and to reduce the pixel pitch size. Having multiple discrete parts in the LED display system also increases the difficulties in controlling other performance parameters such as the timing of the LEDs, the elimination of parasitic capacitance (which may lead to ghost images), etc.